KABUL — Prisoners handed over by international forces to Afghan custody are being subjected to “systematic’’ torture by Afghan interrogators seeking intelligence in the war against the Taliban, according to a United Nations report released Monday.

The report portrays prisoner abuse by Afghan authorities on a scale far wider than previously known. It is bound to complicate American efforts to hand over increasing responsibilities to Afghan forces as U.S. troops begin a steady drawdown from Afghanistan.

The findings include the use of brutal beatings and electric shock at several Afghan-run centers to wrest confessions from detainees. The account, based on interviews with detainees conducted as recently as August, raises questions about whether U.S. officials knew or should have known about abuses involving prisoners turned over to the Afghans.

George Little, a Pentagon spokesman, said the United States is reviewing the “serious allegations” in the U.N. report. The Afghan government challenged the findings, saying that some depictions were “not close to reality,’’ but it also pledged to investigate the allegations of torture and abuse.

“Maybe there are deficiencies with a country stricken by war and a wave of suicide attacks and other terroristic crimes, we do not claim perfection and that we are doing things 100% in accordance with how things should be,” the Afghan government wrote in its response to the U.N. findings, which was included as an annex to the report.

The report reveals serious deficiencies in the Afghan security institutions, primarily the intelligence agency known as the National Directorate for Security (NDS) and the Afghan National Police, organizations that will only gain more responsibility as the U.S. military begins its withdrawal from Afghanistan this year.

The 74-page U.N. report paints a picture of an Afghan detention system routinely and severely abusing its inmates, most of them suspected militants caught at the height of the Obama administration’s surge in Afghanistan. U.S. troops detain thousands of suspected insurgents each year and regularly pass them off to Afghan authorities.

Among the nearly 400 prisoners interviewed by U.N. investigators over an 11-month period that ended in August, 89 said they had been captured by international forces acting either alone or together with Afghan forces. Among the group, the U.N. said it had found “compelling evidence’’ that 22 of them had been tortured in Afghan custody.

Detainees described being hung by their wrists; beaten with rubber hoses, electric cables, wires or sticks; subjected to electric shocks on their bodies and soles of their feet; having toenails removed; enduring “twisting and wrenching” of genitals; and being threatened with sexual abuse, the report says.

Briefed in advance about the U.N. findings, the U.S.-led coalition had halted the transfer of prisoners to several Afghan-run detention centers last month. There has been no indication about when those transfers might resume, leaving U.S. forces in Afghanistan to maintain custody of suspected insurgents captured on the battlefield, at a time when the population of the main American detention center, near the Bagram air base, had already been swelling.

The abusive environment described by the report could also imperil U.S. aid to Afghan security forces under U.S. laws that prohibit the funding and support of a country’s security forces if those forces commit gross human rights violations.

One prisoner, identified as Detainee 371 in the report, said he endured brutal treatment at the intelligence agency’s detention center in Kandahar. “You should confess what you have done in the past as Taliban; even stones confess here,” an interrogator told him, he recalled in the report. After resisting confession for two days, “he tied my hands on my back and start beating me with an electric wire. He also used his hands to beat me.” Another official told the prisoner: “Confess or be ready to die. I will kill you.”

The report raises particular concerns about detention centers run by the Afghan intelligence agency, which held between 1,500 and 2,000 detainees during the period when the investigation took place. It said that nearly half of the detainees held by the intelligence agency had been tortured and that “torture is practiced systematically’’ in “a number’’ of the agency’s detention centers.

More than a third of the 117 conflict-related detainees interviewed who were held by the Afghan National Police also suffered treatment amounting to torture or other cruel, inhumane and degrading treatment, the report says.

A coalition spokesman in Kabul said he did not know how many Afghan detainees have been kept in NATO custody in recent weeks who would otherwise have been transferred to Afghan detention centers. The Bagram facility holds more than 2,500 prisoners, a total that has tripled over the past three years.

U.S. officials intend to expand the prison’s capacity from 3,500 to 5,500 beds to accommodate the growing number of detainees, and the decision to halt transfers to Afghan detention centers could put more pressure on Bagram.

In a statement, the U.S.-led coalition said it has worked in recent weeks to help the Afghan government develop a six-phase plan to reform the detention system in response to the U.N. findings. The coalition began its own inspections at six detention centers and has “begun remediation training at one facility,” according to a military statement.

The coalition “remains committed to eliminating human rights violations in detainee operations,’’ the military statement said.

Salahuddin is a special correspondent. Staff writer Craig Whitlock in Washington contributed to this report.

Joshua Partlow is The Post’s bureau chief in Mexico. He has served previously as the bureau chief in Kabul and as a correspondent in Brazil and Iraq.

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